Method and apparatus for charging and testing batteries

ABSTRACT

An apparatus for charging and testing a rechargeable battery is adapted to determine certain conditions, including defects and characteristics, of the battery. The apparatus detects defects caused by sulfated cells, short circuited cells, and mismatched cells, and determines battery voltage, capacity and charge acceptance capability.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 919,417, filedOct. 16, 1986, now U.S. Pat. No. 4,745,349, entitled "Apparatus andMethod for Charging and Testing Batteries".

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to a method and apparatus, for charging andtesting rechargeable batteries, especially lead-acid batteries of anycapacity and voltage. More particularly, this invention relates to sucha method and apparatus by which the characteristics of the battery, suchas fault conditions are diagnosed in detail, and in which chargingcharacteristics and the battery is charged with maximum efficiency andspeed.

2. Prior Art

Methods and apparatus for determining the true capacity, nominalvoltage, state of charge, defect, gassing, charge acceptance, and othercharacteristics of the battery. Manual determination of these parametersis expensive and time consuming. With the introduction of microprocessorbased autonomous chargers these operating characteristics are determinedautomatically and the battery is charged under optimum conditions.

Typically, the industrial lead-acid batteries have charge capacities ofhundreds of ampere-hours. For maximum utilization, these batteriesshould be recharged quickly for the next use cycle. To prevent damage tothe battery, it should be recharged soon after its used. The batterycharger should be simple to operate, and should warn the user of batterydefects and when to change the battery. It is desired that defective andunsafe conditions of the battery during charge should be indicated.

Some early commercial chargers employed constant charging current for apreset duration. Depending on the charge time available (the time thebattery should be ready for use) and the capacity, the operator sets thecharging current level. The efficiency of this type of charger is low,since state of charge (SOC) is not taken into account. Even with lowinitial SOC, the battery may evolve gas significantly in the lastquarter of the charging cycle.

Some other chargers use constant voltage charging techniques for aspecified time selecting a charge voltage which is lower than the gasvoltage. This method may often lead to undercharging. Theoretically ittakes an infinite time to charge, without gassing, a battery completelyby this method since the charge current decreases asymptotically.

A charger from Westinghouse Devenset Rectifier of England employsanother variation of this technique. The battery is supplied with acharging current until the battery voltage reaches a predetermined levelcorresponding to the gas evolution voltage (Vgas). The charging iscontinued from this stage by a timer for a specified period, followed byan equalizing charge. The battery is then placed under trickle charge tocompensate for the open circuit self-discharge loss. The energy lossduring the timer controlled charge period is still considerable anddetrimental to the battery.

The charger introduced by Oldham/Harmer & Simmons of England passes acharging current to the battery until the voltage rises to the gasevolution voltage. The charger then alternates between a measuring cycleand a charging cycle. In the measuring cycle, the charge current ismeasured while the battery is charged under the constant voltage modecorresponding to the gas voltage. The charge is terminated when thecurrents in two successive measuring cycles are equal.

A charger employing periodic discharge pulses during the charge regimehas been commercialized by Christie Electric Corp. The state of chargeis derived from the current during the discharge pulse. This charger hasbeen designed for small low capacity batteries.

The prior art has also described chargers usingcomputers/microprocessors to perform analytical and control functions.One of the earliest chargers of this type analyzes voltage-current (I-V)characteristics during an applied current ramp to the battery. The I-Vdata is determined for each cell in the battery and compared with theaverage of all the cells. If any cell exhibits significantly differentcharacteristics, the battery is diagnosed as defective. However, forpractical purposes the cells in batteries are often inaccessible.

Another charger of this class uses the slope of the voltage currentcurve, obtained from the I-V characteristics as described above, todetermine the state of charge of the battery. This is accomplished bycomparing the above slope with those of average I-V characteristics ofsimilar batteries at various charge levels (SOCs).

Yet another microprocessor based charger that uses I-V characteristicsof current ramping test cycle has been proposed in EP Nos. 067589 and067590. The I-V characteristics of the batteries of different capacities(within a narrow limit) and states of charge are stored in memory. Theyare compared with that of the battery being charged to determine SOC. Ifno match is found, the charger assumes a fault condition and calls forthe attention of the operator. The battery is charged until the I-Vcharacteristics are almost the same in successive test cycles.

All the chargers proposed in the state of the art are limited tobatteries of certain nominal voltage and capacity within a narrow range.The fault-detecting diagnostics are also limited. For example,mismatched cells and soft-shorted cells are not signaled separately.Clearly, there is a need for a charger that can automatically identifythe operating characteristics of the battery, detect fault conditions,and carry out charging with high efficiency and speed.

SUMMARY OF THE INVENTION

This invention provides an apparatus for charging and testing arechargeable battery, as for example a lead acid battery, of anycapacity and voltage to determine certain conditions, including defectsand characteristics, of the battery. Generally stated, the apparatuscomprises a microprocessor means for controlling the operations of theapparatus. A software means instructs the microprocessor means tocontrol the sequence of the operations. Memory means are connected tothe microprocessor means for storing the software instructions,predetermined data and cell characteristics of the batteries.

A digital to analog converter means is connected to the microprocessormeans for converting digital signals from the microprocess or means toanalog signals. The digital to analog converter means has connectedthereto a direct current power generator means, for producing electricpower at required voltage and current, as commanded by themicroprocessor means. The apparatus has a pair of output means, forconnection to the battery. A current sensor means, connected to theoutput means measures the current passing through the output means toand from the battery. A voltage measuring means measures the voltages ofthe battery, current sensor and direct current power generator means. Ananalog to digital converter means, is connected to the voltage measuringmeans, for converting the analog signals from the voltage measuringmeans to digital signals for transmission to the microprocessor.

A second software means analyzes the current and voltages fordetermining number of cells, capacity, state of charge and defects inthe battery. The apparatus further comprises an actuator means-.. forcontrolling the electrical circuits connecting the current and voltagesensors, the direct current power generator means and the battery. Acontrol means is connected to the power generator means for controllingthe voltage and current supplied by the power generator means to thebattery. A display means, controlled by the microprocessor meansindicates the battery status and the status of apparatus, and advice.

In one aspect of the invention, the power generator means of theapparatus comprises means for generating direct current supplied to thebattery, the value of the current being determined according to apredetermined function of time by the software means in conjunction withthe memory means and voltage measured by said voltage measuring means.

In another aspect of the invention, the power generator means of theapparatus comprises means for generating dc voltage applied to thebattery, the value of the voltage being determined according to apredetermined function of time by the software means in conjunction withthe memory means and current measured by the voltage measuring means.

The software means of the apparatus comprises means for determination ofminimum, maximum and true number of cells in the battery, means forcontrollably varying voltage and current applied to the battery, meansfor determination of gas point of the battery, means for detectingdefect conditions of the battery and means for determining the capacityand state of charge of the battery.

In addition, this invention provides a method of testing a rechargeablebattery, which comprises the steps of:

(a) measuring the open circuit voltage of said battery and estimatingthe number of cells possible for the measured open circuit voltage;

(b) supplying a controllably varying charging current or voltage to thebattery for a predetermined period of time while measuring the responsevoltage or current produced at or through the battery terminals andtesting said battery for the evolution of gas;

(c) charging said battery automatically at any charge rate until thebattery charge voltage equals the estimated number of cells of thebattery multiplied by a predetermined voltage which is characteristic ofthe battery;

(d) repeating steps (a), (b) and (c) until step (b) indicates theevolution of gas;

(e) determining the current ("Igas-up") and voltage ("Vgas-up") at whichsaid battery evolves gas in the increasing current direction, and thecurrent ("Igas-down") and voltage ("Vgas-down") at which the batterystops evolving gas in the decreasing current direction;

(f) determining the true number of cells in the said battery fromVgas-up and/or Vgas-down;

(g) determining the state of charge using the true number of cellsdetermined in step (f), the open circuit voltage measured in step (a)and the charge input to said battery by the apparatus;

(h) determining the capacity of said battery from said Igas-up when saidIgas-down is lower than or equal to a predetermined value, or from thedifference in said Igas-up and Igas-down when said Igas-down is greaterthan or equal to a predetermined value;

(i) determining defect conditions from the open circuit voltage; and

(j) determining defect conditions from the current-voltagecharacteristics generated in step (b).

Another method of this invention comprises the following steps, inaddition to the above steps: PG,8

(a) charging the battery automatically at any charge rate until thebattery voltage equals the true number of cells multiplied by apredetermined voltage which is characteristic of the battery;

(b) charging the battery at a constant voltage equal to voltage in step(a) until the charge current decreases to a predetermined low value;

(c) charging the battery with a predetermined constant current at anyvoltage for a predetermined period of time;

(d) repeating steps (b) and (e) in previous method and step (c) in thismethod until said Igas-down reaches a predetermined lower limit which ischaracteristic of the desired state of charge of the battery; and

(e) determining the battery's capability to accept charge from thecharge input to said battery.

A method of this invention estimates the number of cells in the batteryusing the minimum number of cells estimated by the formula:

    minimum number of cells=(OCV/V.sub.1)

wherein the value of V₁ corresponds to the cell voltage of a completelycharged cell and OCV is the open circuit voltage.

Another method of this invention estimates the number of cells in thebattery using the maximum number of cells estimated by the formula:

    maximum number of cells=(OCV/V.sub.2)

wherein the value of V₂ corresponds to the cell voltage of a completelydischarged cell and OCV is the open circuit voltage of said battery.

A method of this invention determines the true number of cells from themaximum number of cells (MAXCEL) when the estimated number of cells issmaller than a predetermined value.

This invention provides a method to charge the battery until the batteryvoltage equals the minimum number of cells times a predetermined valuecharacteristic of the battery..

A method of this invention determines the true number of cells bydividing Vgas by V₃ wherein V₃ is a value characteristic of the batterytype.

A method of this invention to determine gas point parameters comprisesthe steps of:

(a) monotonously increasing the battery voltage over a predeterminedperiod of time from the open circuit voltage to a predetermined highlimit voltage which is characteristic of the type of battery, holdingsaid voltage at said high limit value for a predetermined period oftime, and monotonously decreasing the battery voltage from said highlimit value over a predetermined period of time to the open circuitvoltage;

(b) measuring the response current and/or impedance during saidincreasing and decreasing voltage;

(c) analyzing the data using the differential dI/dV vs. I and/or V,wherein I is the current and V is the voltage, or dZ/dV vs. I and/or Vwherein I is the current, V is the voltage and Z is the impedance; and

(d) determining the gas evolution parameters by the presence of one ormore minima in dI/dV or dZ/dV in the increasing voltage direction andthe gas stopping point by the presence of one or more minima in dI/dV ordZ/dV in the decreasing direction of said voltage.

Another method of this invention to determine gas point parameterscomprises the steps of:

(a) monotonously increasing the battery voltage over a predeterminedperiod of time from the open circuit voltage to a predetermined highlimit voltage which is characteristic of the type of battery, holdingsaid voltage at said high limit value for a predetermined period oftime, and monotonously decreasing the battery voltage from said highlimit value over a predetermined period of time to the open circuitvoltage;

(b) measuring the response current during said increasing and decreasingvoltage;

(c) analyzing the data using the differential dI/dV vs. I and/or V,wherein I is the current and V is the voltage, and

(d) determining the gas evolution parameters by the presence of one ormore minima in dI/dV in the increasing voltage direction and the gasstopping points by the presence of one or more minima in dI/dV in thedecreasing direction of said voltage.

This invention provides a method of detecting reverse connection of thebattery leads to the apparatus, wherein the measured open circuitvoltage is less than a predetermined voltage, particularly -1 V.

A method of this invention detects improper connections of the batteryto the apparatus using the measured open circuit voltage when it is lessthan a predetermined voltage and higher than another predeterminedvoltage.

A method of this invention detects a defective battery condition whereinthe open circuit voltage is greater than a predetermined voltage andsubstantially zero current flows through the battery in response to anapplied voltage substantially greater than the open circuit voltage. Ondetecting the above conditions, this invention indicates the presence ofone or more of the following defective conditions:

(a) Bad connections;

(b) Corroded terminals

(c) Loss of electrolyte

(d) Bad relay type components in the apparatus

A method of this invention detects a battery having cells mismatched incapacity wherein said mismatch is indicated by the presence of multiplegas evolution points in the increasing current or voltage direction andone or more gas stopping points in the decreasing current or voltagedirection.

A method of this invention detects a battery defect caused by sulfatedcell(s) wherein said defect is indicated by the presence of one or morecurrent-voltage inflections in the increasing current or voltagedirection coupled with the absence of corresponding gas stopping pointin the decreasing current or voltage direction.

A method of this invention detects a battery defect caused bysoft-shorted cell(s) wherein said defect is indicated by adisproportionately low value of Igas-up for the gas evolution points ora jump of a minimum predetermined increase in the open circuit voltageof the battery due to the applied current or voltage ramp to thebattery.

A method of this invention determines the internal resistance of thebattery from two sets of Igas-up and Vgas-up parameters generated fromthe voltage or current ramps, each of said sets corresponding todifferent states of charge of said battery.

A method of this invention detects a defective battery wherein saiddefect is indicated by comparing the determined internal resistance withthe corresponding value characteristic of said battery size and capacitystored in the memory means.

Another method of this invention used to determine the gas pointparameters comprises the steps of:

(a) monotonously increasing the battery current over a predeterminedperiod of time from zero or substantially zero current to apredetermined high limit current and holding said current at said highlimit value for a predetermined period of time and monotonouslydecreasing said current from said high limit value over a predeterminedperiod of time to zero or substantially zero current;

(b) measuring the response battery voltage and/or impedance during saidincreasing and decreasing current;

(c) analyzing the data using the differential dV/dI vs. I or V, whereinI is the ucrrent and V is the voltage, or dZ/dI vs. I or V wherein I isthe current, V is the voltage and Z is the impedance; and

(d) determining the gas evolution parameters by the presence of one ormore maxima in dV/dI or dZ/dI in the

Through use of the method of this invention secondary batteries can becharged with maximum efficiency and speed, without shortening the lifeof the battery by exposure to severe charging conditions. Moreover, themethod of this invention can be used to detect fault conditions in thebattery, and to determine battery capacity, state of charge and nominalbattery voltage. Furthermore, the method of this invention is suitablefor use with batteries of all type and sizes, and can be used withoutthe need for predetermined operational characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed descriptions, given by way of example, will bebest understood in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram of an apparatus capable of carrying out themethod of the present invention.

FIG. 2 is a flow diagram of a preferred embodiment of this invention.

FIG. 3 is a plot of dV/dI versus the ramp current using a current rampwith a battery at 93% state of charge.

FIG. 4 is a plot of dV/dI versus the ramp current using a ramp currentwith a battery at a 100% state of charge.

FIG. 5 is a plot of dV/dI versus battery voltage using a current rampwith a battery at 93% state of charge.

FIG. 6 is a plot of dV/dI versus battery voltage using a current rampwith a battery at 100% state of charge.

FIG. 7 is a plot of dI/dV versus battery current using a voltage rampwith a battery at 50% state of charge.

FIG. 8 is a plot of dI/dV versus battery current using a voltage rampwith a battery at 100% state of charge.

FIG. 9 is a plot, which shows the variation of cell voltage andimpedance as a function of charge time during a constant current charge.

FIG. 10 is a plot which shows the dependency of the rate of change ofbattery voltage on the charge rate.

FIG. 11 is a charge profile of a lead-acid battery whose initial SOC was50% charged with the battery charger of this invention in accordancewith the embodiment of FIG. 2.

FIG. 12 is a flow diagram of a preferred embodiment of this invention.

FIG. 13 is a charge profile of a lead-acid battery, whose initial SOCwas 50%, charged with the battery charger of this invention inaccordance with the embodiment of FIG. 12.

FIG. 14 is a series of plots determining gas point by a voltage rampmethod.

FIG. 15 is a series of plots showing the detection of the gas point by acurrent ramping method.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Charger 80 illustrated in FIG. 1 consists of a microprocessor 10 andpower unit 30 attached to a battery 40. Microprocessor 10 and power unit30 may be put together in one unit as an "intelligent" power supply ormay exist as two individual units interfaced with appropriate softwareand hardware. Microprocessor 10 sends commands 20 to power unit 30 andthus control its performance and operating characteristics.Microprocessor 10 is controlled by the software incorporated in itsmemory. Microprocessor 10 also analyzes data 50 received from power unit30 and determines appropriate action and further course of the chargingprocess.

Power unit 30 merely functions as a slave to microprocessor 10 andoutputs current and/or voltage as commanded by microprocessor 10. Inaddition to the power output (voltage/current), power unit 30 preferablycan close and open the charging circuit. Power unit 30 preferably alsohas diodes to protect the battery discharging into power unit 30. Powerunit 30 preferably has subassembly resembling voltmeter which is capableof measuring battery voltage and the charge current through a shunt orsimilar device. There is no difference in the hardware connections forthe ramp cycle and the charge cycle. Their difference is only in thesoftware commands sent by microprocessor 10 to power unit 30 and hencethe power (voltage and/or current) output. Power unit 30 preferably hasa digital to analog converter 60 which converts digital commands 20received from microprocessor 10 into analog signals that control thefunctions of the power unit. Power unit 30 also has an analog to digitalconverter 70 which transforms the analog data from power unit 30 todigital data 50 and sends the same to microprocessor 10. The systemconsisting of microprocessor 10, the software and power unit 30 with allits accessories hereafter is referred to as the charger 80.

Power unit 30 has a starter switch (not shown) which is switched on tostart the charging process after battery 40 is connected to charger 80.In addition, power unit 30 has a manual/auto switch (not shown). Inmanual position, charger 80 allows the operator to fix the chargingcurrent and voltage and also permits the user to control all thefunctions of charger 80 manually. In the auto position, charger 80controls all functions automatically according to the batterycharacteristics, detects faulty conditions, if any, and charges mostefficiently and quickly.

Battery 40 is any type of rechargeable battery such as nickel-cadmium,nickel-hydrogen, lead-acid, nickel-zinc, nickel-iron, silver-zinc, zincbromine, zinc-chlorine and the like. However, in the preferredembodiments of the invention, lead-acid batteries are used.

The following description of the drawings which refer to any type oflead-acid battery, is for illustration purposes only, and should not beconstrued as limited to the said battery type. It is appreciated,however, that certain parameters as for example the cell voltagecorresponding to completely discharged and charged states, and gasvoltage will have different values, depending on the specific type ofbattery, and should be properly incorporated into the software commands.The lead-acid battery consists of plurality of cells depending on thevoltage requirement of the application. Each lead-acid cell has avoltage of about 2.2 V when fully charged and a voltage of 2 V whenfully discharged. These values can be higher or lower depending on thepolarization or the rest period after the battery's last charge or use.The cell voltage lies between these values for intermediate states ofcharge. The capacity of the battery can be of any value ranging from afew Ah to hundreds or even thousands of Ah depending on the application.

All control and measurement functions are initiated and commanded bymicroprocessor 10 and they are executed by power unit 30. For example,to generate a linearly increasing voltage ramp, microprocessor 10initially sets up a limit for the voltage and current output. Dependingon the slope of the voltage ramp, resolution and total response times ofmicroprocessor 10 and power unit 30, microprocessor 10 periodicallysends commands 20 to power unit 30 requiring the appropriate voltageoutput. Power unit 30 then waits for a predetermined period, measuresthe response current and passes data 50 to microprocessor 10, which thencommands power unit 30 to output the next required voltage level. Thissequence continues until the set limit of the voltage or current isreached, whichever happens earlier. During the above sequence, thecurrent passing through the battery 40 is predominantly determined bythe battery's characteristics, as for example battery capacity, internalresistance and state of charge.

A flow chart of a predetermined method of operating charger 80 is shownin FIG. 2. When battery 40 is connected to charger 80 and started, allvariables such as the number of cells (NOC), state of charge (SOC),maximum (MAXCEL) and minimum (MINCEL) number of cells, battery capacity(CELCAP), gas currents (I gas-up, I gas-down), gas voltages (V gas-up, Vgas-down), charge current, and charge voltage (CMV) are initialized.

Determination of the true open circuit voltage of the battery is thefirst step in the method shown in FIG. 2. The voltage measured acrossthe terminals of battery 40 when no current is passed to or from battery40 is normally known in the art as the open circuit voltage (OCV) of thebattery. Soon after a lead-acid battery is discharged or charged, thevoltage measured across the battery terminals changes even though thebattery is in open circuit, due to the non-uniformity of the electrolyteconcentration in the pores of the electrodes and the bulk, normallyknown as the concentration polarization. The polarization decreases withtime due to diffusion and convection of the electrolyte between the bulkand the electrode pores and hence the voltage between the terminalsapproaches a constant value reflecting its state of charge which is ameasure of the available energy in the battery in terms of its totalcapacity.

Charger 80 determines the true open circuit voltage (OCV) by measuringthe battery terminal voltage in open circuit repeatedly for apredetermined period as for example, 1 to about 10 minutes, at aconvenient interval as for example (1 to about 10 seconds) andextrapolating to a long interval as for example, 2 to about 4 hours. Themeasured battery voltage (V) varies with log time (log t) linearly. Themathematical equation for this straight line relationship between V andlog t (V=m logt+c, where m is the slope and c is intercept) isdetermined using the data measured for a few minutes as mentioned above.It is obvious that the open circuit voltage after completedepolarization is obtained by inserting a generally accepted time forrelaxation (2 or more hours) in the said equation and evaluating V.Various characteristics of the battery can be determined from the trueopen circuit voltage. For example, the state of charge (SOC) oflead-acid batteries can be determined from the open circuit voltage. Theopen circuit voltage of a battery increases linearly as its state ofcharge increases from 0 to 100%.

For example, the open circuit voltage of lead-acid cell varies linearlyfrom 2 to 2.2 V as its state of charge increases from 0 to 100%. Thestate of charge (SOC) can be determined from open circuit voltageseveral times during the course of charge process using the equation:

    SOC=((OCV/NOC)-2.0)×100/0.20                         (1)

This equation is based on the OCV excluding the electrode concentrationpolarizations. However, the measured OCV invariably includes electrodepolarization. The true OCV can be obtained by using different values forthe denominator to account for the polarization at the time of OCVmeasurement. For example, when the OCV measurement is made 5-10 minutesafter placing the battery in open circuit the denominator 0.26 can besubstituted for 0.20. Thus, the following equation may be used in thiscase.

    SOC=((OCV/NOC-2.0)×100/0.26)                         (2)

Usually, since the battery is relaxed between the time of use and thestart of charging time to determine the initial OCV measurement equation1 may be used. This is not true in other mid-test cycles during thecourse of charging and equations such as the equation 2 can be used. Thevoltage due to concentration polarization can be eliminated by lettingthe electrode equilibrate with the bulk electrolyte for a few hours.Since it is not practical to wait such a long period, especially whenmost of the depolarization occurs in the first few minutes, a 5 to about10 minute waiting period and the equation with larger denominator of0.26 can be used to take care of the remaining polarizations. Thepreferred method for determining the state of charge is the method basedon the battery voltage and log t using equation (1) as describedearlier.

Having determined the OCV of the battery 40, microprocessor 10 evaluatesbattery 40 for some possible defect conditions. The operator is warnedby an alarm or flash for appropriate action, if any fault conditionrequiring the operator's attention is found. Several fault conditions ofbattery 40 can be detected using the method and this testing apparatus.If the measured OCV is less than -1 V, it indicates that battery tocharger connections are reversed If the OCV is between +1 and -1 V, nobattery 40 is connected to the charger 80 or the connections are verypoor. If the OCV is more than +1 V, but no current flows when thecurrent or voltage supplied to battery 40 by power unit 30 is increased,it is indicative of high resistance in the circuit This may arise fromany of the following factors including bad connections, corrodedterminals, loss of electrolyte, bad relay connections in thetester/charger and very heavily sulfated cells.

In the next step of the method of FIG. 2, the OCV is fed intomicroprocessor 10 which then determines the minimum (Minicell) andmaximum (Maxicell) number of cells possible for the OCV of battery 40,and thus fixes the range of the number of cells in battery 40.Microprocessor 10 is programmed such that the upper and lower limits ofthe number of cells are calculated from the open circuit voltage (OCV)using the formula

    Maxicell=(OCV/m)                                           (3)

    Maxicell=(OCV/P)                                           (4)

wherein the constants p and m are characteristics of the type ofbattery, and correspond to the cell voltage of a completely charged celland the cell voltage of a completely discharged cell, respectively. Inthe case of the preferred lead-acid battery, m is equal to 2.0 and p isequal to 2.2. In the practice of the preferred embodiments of thisinvention where battery 40 is a lead-acid battery, if the maximum numberof cells is determined to be less than or equal to eight, then theminimum and true number of cells are set equal to the maximum number ofcells.

The maximum number of cells is normally calculated only at thebeginning, prior to battery charging/testing. This value generallyrepresents the correct cell number if the initial state of charge isless than about 30%. This value is the upper limit in any case. Theminimum number of cells is calculated each time when the OCV is measuredjust before a ramping sequence described below. This usually representsthe correct cell number if the state of charge is greater than about75%. Thus, even if the initial state of charge is low, as the batterygets fully charged, the minimum number of cells converges to the correctvalue. It represents the lower limit, and this limit increases as thebattery is charged to full capacity.

Charger 80 assumes that the number of cells in the battery is equal tothe minimum number of cells and supplies a controllably varying chargingcurrent or voltage to the battery for a predetermined period of time,and tests the battery for the evolution of gas to determine the gaspoint. This procedure is described throughout the specification asramping.

Several pertinent pieces of information can be determined from the gaspoint, which are useful in charging the battery. For example, during thecharging process, how high a current a battery can accept withoutevolving gas depends on its capacity and SOC. For a battery of givencapacity, the threshold current at which the battery starts evolving gasdecreases with increase in SOC.

The voltage at the gas point (Vgas) is used to determine the number ofcells in the battery using the formula:

    Number of cells=Vgas/n                                     (5)

The denominator n depends on charger 80, the type of battery and therate of change in the applied parameter (current or voltage). In thecase of lead-acid batteries, n is in the range 2.35-2.60V, and isusually 2.50V. The gas current in the increasing direction (Igas-up),when the battery is fully charged is characteristic of the batterycapacity, and can be used to determine the same. The gas current in thedecreasing direction, Igas-down, reaches the lowest possible value for agiven charger 80, battery 40 and slope of the ramp, and is used todenote the end of charge. The difference in the gas current of eitherdirection (Igas-up--Igas-down), is also useful to determine theapproximate battery capacity at any state of charge.

Aside from the above method based on Igas-up, an alternate method basedon the amount of charge the battery has accepted (ΔC) and thecorresponding increase in its state of charge (ΔSOC), can be used todetermine the battery Capacity (C) according to C=ΔC×100/ΔSOC).

The battery capacity can also be determined from dV/dt of step 1 of thecharge curve. For a given constant charge current, dV/dt is inverselyproportional to the capacity. In this method, dV/dt is measured for agiven constant charge current in step (g). The corresponding charge rateis determined from a previously collected data and dV/dt is measured atvarious charge rates which data is stored in the memory ofmicroprocessor 10. The current times the charge rate is equal to thebattery capacity. Alternate methods for determining the battery capacityinvolve the calculation of C in the ramp cycle from the maximum indV/dI, or from the minimum in dI/dV at 100% SOC, or from thedepolarization curve (V-t) at 100% SOC.

Microprocessor 10 can be programmed to determine the battery capacity byany of these methods. However, the Igas-up method of determining thecapacity of the battery is preferred.

Any method of determination of the gas point and the various parametersattended thereto can be used in the method of this invention. In thepreferred embodiment of the invention, three methods may be used todetermine the voltage and current at the threshold level called the gaspoint. One useful method is to ramp the current up to a maximum valueand then down to zero. Typically, when the current ramping method isused, the current signal is linearly increased from 0 to a predeterminedvalue over a predetermined period of time. The current is maintained atthe predetermined level for a predetermined period and then decreased to0 over another predetermined period of time, which is usually the sameperiod of time over which the current is increased. It is appreciatedthat predetermined upper limit of the ramping current and the rampingtime is dependent upon the output capability of power unit 30, responsetimes of microprocessor 10 and power unit 30, and the capacity ofbattery 40. For example, typically when the current ramping method isused with lead-acid batteries the current signal consists of anincreasing ramp from 0 to about 20A, over a period of about 20 to about60 seconds, the current is held constant at this level for about 0 A in20-60 seconds. During the ramp, the voltage response of the battery ismeasured. A plot of dV/dI vs. Iramp, dV/dI vs t, (the ramp time),exhibits a maximum corresponding to the gas point as shown in FIG. 3.The gas point appears at a higher value of current in the increasingramp direction compared with that in the decreasing direction. However,Vgas, the voltage at the gas point remains about the same in eitherdirections. FIGS. 3 and 4 show dV/dI vs Iramp plots for the same batteryat two different states of charge (93 and 100%). It is noteworthy thatthe peaks move to lower values of current as SOC increases. At 100% SOC(FIG. 4), the Igas-down has reached the lowest possible (set) value,while the Igas-up remains at a higher value. The latter is useful incalculating the battery capacity as discussed before under thesignificance of gas point. FIGS. 5 and 6 exhibit the corresponding dV/dIvs V response data. It is important to note that the voltage where amaximum occurs does not change appreciably with SOC, and that it ischaracteristic of the number of cells in the battery.

Another method used to determine the voltage and current at thethreshold level called the gas point is the voltage ramping method. Whenthe voltage ramping method is used, the signal consists of a continuousor discrete increasing voltage ramp from the OCV to a predetermined endvoltage usually a voltage corresponding to about 0.4 to about 0.6 Vhigher than the OCV for each cell. During the voltage ramp, the currentresponse of the battery is measured. A plot of dI/dV vs Vramp, Iresponseor ramp time exhibits a minimum corresponding to the gas point. Thepresence of multiple minima in any direction indicates the presence ofmismatched cells in the battery. FIGS. 7 and 8 show dI/dV vs. Iresponsedata for the same battery at different states of charge (50 and 100%).The Igas and Vgas parameters have the same significance as in theprevious current ramp method. In the case of both the current rampingmethod and the voltage ramping method, the increasing signal changesover to the holding portion earlier than the time limit, 60 seconds,whenever the voltage limit (2.5-2.8 V/Cell for lead-acid batteries) isreached. It is appreciated that this situation would be encountered whenthe battery capacity is low and/or its state of charge is high.

The presence of a maximum in dV/dI versus Iramp or a minimum in dI/dVversus Vramp indicates the gas point. In the increasing ramp direction,the battery starts evolving gas at the gas point, and in the decreasingramp direction the battery stops evolving gas at the gas point. From thevoltage at the gas point, the true number of cells (NOC), is calculatedusing the formula:

    NOC=Vgas/n                                                 (6)

wherein n is a value which is characteristic of the type of battery. Thedenominator is the cell voltage at which gas evolution starts in thedynamic conditions of the ramp, and is dependent upon the type ofbattery and slope of the ramp, and falls generally in the range fromabout 2.35 to about 2.65 V/cell for lead-acid cells. The cell voltage atwhich gas evolution occurs in other types of secondary batteries willdiffer which will necessitate a change in the denominator.

Another method of determining the gas point is by impedance. When abattery is charged under constant current, the variation of batteryvoltage and impedance with the charge time is shown in FIG. 9. Thesimilarity between the two curves, especially the sharp increase in bothparameters at the gas point, suggests that the gas point may be detectedby measuring the impedance, particularly its real part, while rampingthe current or applied voltage. Thus, a method based on ramping currentor voltage and measuring the battery impedance (Z), and voltage orcurrent can also be used to detect the gas point. In this method, an acsignal whose frequency is equal to or less than 1 Hz is used to reasurethe impendance of the battery. A plot of dZ/dI vs. I, V or t hascharacteristics very similar to that of dV/dI vs. the correspondingparameter. For example, a plot of dZ/dI vs. I is similar to FIGS. 3 and4. Likewise, Igas and Vgas are obtained from the parameters at themaximum point.

A number of defects of the battery can be determined from the gas pointcharacteristics. If two or more peaks are observed on the increase involtage or current (ramp up direction) along with the presence of atleast one peak in the decreasing direction (ramp down), then battery 40has cells mismatched in capacity. The observations of one or more peaksin the ramp up direction, coupled with their absence in the ramp downdirection, indicates that battery 40 (if a lead-acid battery) issulfated. If the internal resistance as determined is higher than a setlimit, the charger/tester activates a warning signal. The presence of asoft-short circuited cell is indicated if mismatched cells are foundwith one or more cells having very low capacity (a disproportionatelylow value for one of the Igas-up parameters) or if a jump of at least1.5 V in open circuit voltage occurs due to ramping (OCVs before andafter ramp differ by 1.5 V or more).

When a high voltage battery having many cells in series is chargedpossibly at high current, the voltage drop due to internal resistance ofthe battery can be substantial. In such cases, it is important toestimate the internal resistance. Vgas is characteristic of the numberof cells in the battery, and should be constant irrespective of SOC andcapacity. In practice, Vgas varies to some extent at high values ofIgas, which is due to the internal resistance. The charger and method ofthis invention can thus determine the battery's internal resistance (R)from two ramp cycles as follows:

    R=(Vgas-up1--Vgas-up2)/(Igas-up1--Igas-up2)                (7)

If the current or voltage ramp test indicates the r presence of a gaspoint then battery 40 is charged under constant current chargeconditions preferably at a current that is equal to the maximum currentthat power unit 30 can deliver (or according to the battery capacity)subject to a high voltage limit set at Vgas. During this constantcurrent charge period the battery voltage increases and reaches Vgas atwhich time the battery is charged in accordance with constant voltagecharge mode.

If the ramp does not indicate the presence of a gas point, the number ofcells is assumed to be equal to a new minimum number of cells calculatedin using the new OCV and equation 4 and the battery is charged underconstant pre-determined current, usually equal to the maximum currentthat power supply 30 can deliver (or according to the battery capacity)until the battery voltage reaches a value equal to the minimum estimatednumber of cells multiplied by 2.5. Then it is subjected to the ramp todetermine the gas point. This procedure is repeated until rampingindicates gas evolution.

If the maximum charge current is desired to be based on the batterycapacity or at a specific charge rate (e.g. 5 or 6 hour rate), it can beimplemented using the dV/dt of the charge curve. The relationshipbetween the rate of change of battery voltage and the charge rate isshown in FIG. 10. The battery is charged at a predetermined rate, whichcan be any desired rate, by making charger 80 adjust the charge currentcontinuously such that dV/dt is maintained at the corresponding valuegiven by the data in FIG. 7. Processor 10 automatically determines thelevel of the charge current depending on the desired charge rate set bythe operator. If the operator has not selected a charge rate, processor10 assumes the default value of 6 hour charge rate. The relationshipbetween the charge rate and the rate of increase of a lead-acid cellvoltage, when no gas is evolved, is shown in FIG. 10. The data of thisfigure is stored in memory. Processor 10 adjusts the charge current suchthat the rate of change of the battery voltage conforms to the valuefound in the said data in memory corresponding to the selected chargerate.

During this constant current charge period, the battery voltageincreases and reaches the said limit value. Since processor 10 does notlet the charge voltage rise beyond the said limit, the charge currentdecreases. When the said current falls to a predetermined percentage ofthe constant charge current, the microprocessor 10 opens the chargingcircuit. Battery 40 is kept in open circuit for a predetermined periodas for example to about 10 minutes. The true open circuit voltage andthe minimum number of cells (MINCEL) are determined as described above.The number of cells is made equal to the new MINCEL value. It isappreciated that the said new MINCEL value may be larger than theprevious MINCEL value if the state of charge of the battery has changedsubstantially and the previous MINCEL value is lower than the truenumber of cells.

After the open circuit period, microprocessor 10 performs another rampcycle (step b) and checks for the presence of gas point and batterydefects. This sequence of charging until the charge current becomeslimited by voltage limit and subjecting the battery to a ramp cycle isrepeated until the presence of a gas point is detected in a ramp cycle.The correctness of the number of cells is confirmed from the gas pointin the ramp cycle. It is appreciated by those familiar in the art thatone or more of the ramp cycles may be skipped, if desired, since thecharge process is controlled primarily by the MINCEL parameter which isdetermined from the new OCV and not necessarily from the ramp cycle.

It is further appreciated that the values of MINCEL and NOC tends toincrease and reach, but never exceed the value of true number of cells,especially when the initial state of charge of the battery is low. Afterthe detection of the gas point, the true number of cells is calculated,and thereafter, if required, the battery is charged at a constantpre-determined current until the battery voltage reaches Vgas.

In the next step of this method, the battery charging continues in theconstant voltage charging mode with the voltage set at NOC times somevalue which is characteristic of the type of battery. With lead-acidbatteries this value is from about 2.35 to about 2.6 V, preferably 2.5V/cell. This charging at constant voltage is continued until the chargecurrent falls to a predetermined low limit, for example 0.5-1A. Duringthis step of the method, the charge current is allowed only to decrease.Processor 10 does not permit the current to increase even if it requiresa decrease in the charge voltage.

At this time, microprocessor 10 starts charging the battery under thelow limit constant current mode letting the voltage go higher than thesaid limit for a predetermined period of time. The battery is subjectedto the ramp cycle after a predetermined period of time as for example,30 minutes in stage 3 and again at predetermined periodic intervals, forexample 20-50 minutes. The battery is then charged at constant current(preset low limit) until at least one of the following conditions aresatisfied: (a) The charging current is above zero but equal to or lowerthan the preset low limit (b) I gas-down reaches a predetermined valuecharacteristic of the charger system and slope of the ramp; or thedifference between two successive I gas-down or I gas-up values becomesless than or equal to a set limit, for example 1 mA. The charge profileis shown in FIG. 11 for a battery whose initial SOC was 50%.

Finally, the end of charge message, the capacity calculated from the gasevolving current, I gas-up, and the nominal battery voltage aredisplayed.

Another method of this invention is depicted in FIG. 12. In thisembodiment, the number of cells is initially set equal to the maximumnumber of cells (MAXCEL). If the initial state of charge of the batteryis low, MAXCEL may actually be equal to the true number of cells. If theinitial state of charge of the battery is high, MAXCEL may be greaterthan the true number of cells. It is appreciated that MAXCEL is neverlower than the true number of cells in the battery. MAXCEL is determinedonly at the beginning of the charge process.

A flow chart of this method is shown in FIG. 12. When battery 40 isconnected to charger 80 and started, variables as for example numbercells(NOC), state of charge (SOC), maximum (MAXCEL) and minimum (MINCEL)number of cells, battery capacity (CELCAP), gas currents (Igas-up,Igas-down) and voltages (Vgas-up, Vgas-down), charge current and voltage(CMV) are initialized. Charger 80 measures the open circuit voltage(OCV) of battery 40. Some faulty conditions like reverse, bad or noconnections are detected, and the user is warned by an alarm or a flash.Charger 80 determines at least the maximum number of cells possible forthe measured OCV. Charger 80 assumes the number of cells to be themaximum and goes through a ramp cycle. The voltage limit for the rampcycle is set at some voltage which is characteristic of the battery. Forexample, in a lead-acid battery, the voltage limit is preferably 2.6-2.8V times the MINCEL. In the ramp cycle, either current ramping method orvoltage ramping method, as described earlier, is employed and the gaspoints are determined. From the voltage at the gas point, the number ofcells (NOC), is calculated using the formula:

    NOC=Vgas/n                                                 (8)

wherein n is a constant which is characteristic of the particularbattery being charged. For example, with lead-acid batteries, n is2.50V. In the absence of a gas point in the ramp cycle, the assumedvalue of NOC continues to be in effect until the actual value isdetermined from a succeeding ramp cycle. The battery is then charged fora predetermined interval e.g. 20 to 60 minutes at the maximumpermissible constant current subject to a high voltage limit at Vgas, orV=n×NOC. The charging cycle interval may be varied depending on thestate of charges (SOC). For example, the higher the state of charge, theshorter the charge interval, and conversely, the lower the state ofcharge, the longer the charge interval. The battery is placed in opencircuit for a predetermined period of time, e.g. 1 to 10 minutes afterwhich another ramping cycle starts. As an alternative to this long opencircuit period, voltage-time data collected for a very short period,typically a minute or less, can be extrapolated to determine the OCV atinfinite time as shown in FIG. 3. This sequence of charging and rampingis repeated until the end of charge. However, when the above voltagelimit is reached, the charge process continues at constant voltage untilthe current falls to a preset low limit e.g. 0.8A. Then the battery ischarged at this low constant current letting the voltage float. The endof charge is detected as described earlier. Battery capacity calculatedfrom the Igas-up, the nominal battery voltage, and completion of chargeare displayed. A typical charge profile which was charged with thecharger of this invention in accordance with the embodiment depicted inFIG. 12 and whose initial SOC was 50% is shown in FIG. 13.

One advantage of the method and apparatus of this invention is that thebattery undergoes test cycles only a few times during the entire chargeprocess, even if the initial state of charge is near zero. This may leadto shorter charge times. In the above methods, the open circuit periodshelp reduce polarization, and as a result the battery experiences lesssevere charging conditions leading to high charger efficiencies. Inaddition, since the method and tester of this invention can detectfaulty conditions of the battery, they can be used as quality controlinstrument in battery manufacturing plants, functioning as a batterylester. This embodiment of the invention can also be adapted to functionas a battery burn-in apparatus (cycles the battery a few times beforeshipping to retailers) in battery manufacturing companies.

The following specific examples are presented to more particularlyillustrate the invention and are not to be construed as limitationsthereon.

EXAMPLE I Voltage Ramping

Attention is directed to FIG. 14 which shows a variation of the appliedvoltage signal, the current response and the differential, dI/dt, withtime, t, of a 4-cell 20 Ah battery at different states of charge. At thegas point, the current lags the voltage and consequently thedifferential exhibits a minimum. Igas-up and Vgas-up refer to thecurrent and voltage at the gas (minimum) point in the ramp-up direction,while Igas-down and Vgas-down refer to the corresponding parameters inthe ramp-down direction. A plot of the Igas-down versus the state ofcharge leads to the gas curve. The gas curve indicates the maximumcurrent the battery can accept without evolving gas. Therefore, thebattery charging can be accomplished in the shortest possible time andmost efficiently by making the charge current follow the gas curve asclosely as possible. The method of this invention accomplishes this whena charge voltage limit for stage 1 and the constant charge voltage forstage 2 are chosen in the range of 2.35 to 2.60 V per cell, particularlyaround 2.48 V per cell. (See FIG. 13).

EXAMPLE II Current Ramping

Attention is directed to FIG. 15, which shows variation of the appliedcurrent signal, the voltage response and the differential dV/dt withtime, t, of the same 4-cell 20 Ah battery as in Example I (FIG. 14), atdifferent states of charge. At the gas point, the voltage signalincrease before the current signal and consequently, the differentialexhibits a maximum. The Igas-up, Igas-down, Vgas-up and Vgas-downobtained from FIG. 15 are comparable to the corresponding parameters inFIG. 14 (Table 1).

                  TABLE 1                                                         ______________________________________                                        Gas point parameters by current and                                           voltage ramping methods.                                                      Soc        I gasup  I gasdown V gasup                                                                              V gasdown                                ______________________________________                                        I ramp  100    2.06     0.48    10.30  10.07                                          80     3.33     2.22    10.23  10.21                                          70     5.24     4.45    10.30  10.39                                          60     8.10     7.79    10.48  10.63                                  V ramp  100    1.68     0.63     9.79  10.06                                          80     3.39     2.43    10.01  10.39                                          70     5.79     4.58    10.53  10.52                                          60     9.15     8.15    10.90  10.95                                          50     13.16    12.18   11.03  11.09                                  ______________________________________                                    

EXAMPLE III Charging with different initial SOC

A 20 Ah battery was discharged to various known depths, and thenrecharged with the apparatus of this invention. The charge output fromthe battery during discharge, the corresponding state of charge, thesubsequent charge input to the battery during the charge process, andthe percent of the charge wasted are shown in TABLE 2.

                  TABLE 2                                                         ______________________________________                                        Charge Details at different                                                   initial states of charge.                                                     Discharge Charge     Initial Soc                                                                             % Wast*                                        (Ah)      (Ah)       (%)       (of charge)                                    ______________________________________                                        19.75     20.20       0        1.0                                            15.87     16.70      24        4.9                                            11.21     11.60      44        3.4                                            8.07       8.90      60        9.4                                            3.38       4.03      83        16.0                                           0          0.36      100       100.00                                         ______________________________________                                    

EXAMPLE IV Testing Different Type Of Batteries

Lead-acid batteries of flooded type (excess electrolyte with capacitiesof 20 Ah and 5o Ah, a sealed (starved) lead-acid battery of 100 Ah, aSLI battery of 34 Ah, and a motorcycle battery of 5.5 Ah were dischargedto known depths. They were recharged successfully with the charger ofthis invention. Possible defects like soft-shorted cells in themotorcycle battery were indicated by the charger.

EXAMPLE V Mismatched Cells

A 10 Ah and a 12 Ah batteries were connected in series and charged withthe charger/tester of this invention. Mismatched cells were indicated.In another instance 14-cells of 20 Ah and a 1-cell 50 Ah battery wereconnected in series and charged as one battery. The charger/testerindicated the presence of mismatched cells.

EXAMPLE VI Sulfated Cells

A 20 Ah battery was discharged to the low cut-off limit of 1.75 V/cell,and left in open circuit for 3 days. After this period when charged withsaid charger, the message, "Presence of sulfated cells" was indicated bythe charger, yet the battery was charged successfully. The sameprocedure was repeated with an open circuit period of 8 days afterdischarge with the same result.

EXAMPLE VII Soft-shorted Cells

The presence of soft-shorted cells is one of the common failure modes oflead acid battery. Generally the shorted cell behaves like a normal cellwith low capacity during charge, but becomes a dead cell (loses voltage)during discharge and in long open circuit periods. The time when thecell gets activated during charge is unpredictable. To some extent, thistime of activation depends on how hard the short is and the state ofcharge.

Our test on this aspect were carried out using a 5 Ah 6 V motor cyclebattery. Experiments were done at various initial SOC. At high initialSOC, the soft-shorted cell became active during the first test (ramp)cycle. With he lower initial SOC, the cell got activated only during thecharge cycle. Nevertheless, the charger detected and signaled thepresence of the soft-shorted cell.

EXAMPLE VIII No Connection

The charger was started without connecting any battery. The result wasthe message by the computer, "No battery connected".

EXAMPLE IX High Resistance

During the series of experiments, the relay contacts developed a highlyresistive film. This was indicated by the charger. Also when theterminals are highly corroded and connected to the charger withoutcleaning them, the high resistance is noted. In both instances thefollowing message was flashed by the computer.

"Check water in battery"

"Check relay contacts"

"Check terminal connections"

EXAMPLE X Reverse Connections

The positive terminal of the battery was connected to the negative inthe charger, and the negative terminal of the battery to the positive ofthe charger. The charger warned of reverse connections.

What is claimed is:
 1. A method of charging and testing a rechargeablebattery which comprises the steps of:(a) measuring the open circuitvoltage of said battery and estimating the number of cells possible forthe measured open circuit voltage; (b) supplying a controllably varyingcharging current or voltage to the battery for a predetermined period oftime while measuring the response voltage or current produced at orthrough the battery terminals and testing said battery for the evolutionof gas; (c) charging said battery automatically with a charge input atany charge rate until the battery charge voltage equals the estimatednumber of cells of the battery multiplied by a predetermined voltagewhich is characteristic of the battery; (d) repeating steps (a), (b),and (c) until step (b) indicates the evolution of gas; (e) determiningthe current ("Igas-up") and voltage ("Vgas-up") at which said batteryevolves gas in the increasing current direction, and the current("Igas-down") and voltage ("Vgas-down") at which the battery stopsevolving gas in the decreasing current direction; (f) determining thetrue number of cells in said battery from said Vgas-up and/or saidVgas-down; (g) determining the state of charge using the true number ofcells determined in step (f), the open circuit voltage measured in step(a) and the charge input to said battery by the apparatus; (h)determining the capacity of said battery from said Igas-up when saidIgas-down is lower than or equal to a predetermined value, or from thedifference in said Igas-up and Igas-down when said Igas-down is greaterthan or equal to a predetermined value; (i) determining defectconditions from said open circuit voltage; and (j) determining defectconditions from the current voltage characteristics generated in step(b).
 2. A method as recited in claim 1, further comprising the stepsof:(a) charging said battery automatically at a predetermined chargerate until the battery voltage equals the true number of cellsmultiplied by a predetermined voltage which is characteristic of thebattery; (b) charging said battery at a constant voltage equal tovoltage in step (a) until the charge current decreases to apredetermined low value; (c) charging said battery with a predeterminedconstant current at any voltage for a predetermined period of time; (d)repeating steps (b) and (e) in claim 1 and step (c) in claim 2 untilsaid Igas-down reaches a predetermined lower limit which ischaracteristic of the desired state of charge of the battery; and (e)determining the said battery's capability to accept charge from thecharge input to said battery.
 3. A method according to claim 2 whereinsaid battery is a lead acid battery.
 4. The method according to claim 1wherein the estimated number of cells in the battery is the minimumnumber of cells estimated by the formula:

    minimum number of cells=(OCV/V.sub.1)

wherein the value of V₁ corresponds to the cell voltage of a completelycharged cell and OCV is the open circuit voltage.
 5. The methodaccording to claim 1 wherein the estimated number of cells in thebattery is the maximum number of cells estimated by the formula:

    maximum number of cells=(OCV/V.sub.2)

wherein the value of V₂ corresponds to the cell voltage of a completelydischarged cell and OCV is the open circuit voltage of said battery. 6.The method according to claim 1 wherein the maximum number of cells(MAXCEL) is the true number of cells when the estimated number of cellsis smaller than a predetermined value and wherein the battery is chargedin step (c) of claim 1 at a voltage corresponding to the MAXCEL.
 7. Themethod according to claim 4 wherein the battery is charged until thebattery voltage equals the minimum number of cells times a predeterminedVoltage.
 8. A method according to claim 4 wherein the true number ofcells is determined by dividing Vgas by V₃ wherein V₃ is a voltagecharacteristic of the type of battery. V₃ is a value from about 2.35 toabout 2.60 V for lead acid batteries.
 9. A method according to claim 8wherein V₃ is 2.5 V for lead acid batteries.
 10. A method according toclaim 1 wherein steps (b) and (e) comprise:(a) monotonously increasingthe battery voltage over a predetermined period of time from the opencircuit voltage to a predetermined high limit voltage which ischaracteristic of the type of battery and holding said voltage at saidhigh limit value for a predetermined period of time and monotonouslydecreasing the battery voltage from said high limit value over apredetermined period of time to the open circuit voltage; (b) measuringthe response current and/or impedance during said increasing anddecreasing voltage; (c) analyzing the data using the differential dI/dVvs. I or V, wherein I is the current and V is the voltage, or dZ/dV vs.I or V wherein I is the current, V is the voltage and Z is theimpedance; and (d) determining the gas evolution parameters by thepresence of one or more minima in dI/dV or dZ/dV in the increasingvoltage direction and the gas stopping points by the presence of one ormore minima in dI/dV or dZ/dV in the decreasing direction of saidvoltage.
 11. A method according to claim 1 wherein steps (b) and (e)comprise:(a) monotonously increasing the battery voltage over apredetermined period of time from the open circuit voltage to apredetermined high limit voltage which is characteristic of the type ofbattery and holding said voltage at said high limit value for apredetermined period of time and monotonously decreasing the batteryvoltage from said high limit value over a predetermined period of timeto the open circuit voltage; (b) measuring the response current duringsaid increasing and decreasing voltage; (c) analyzing the data using thedifferential dI/dV vs. I and/or V, wherein I is the current and V is thevoltage; and (d) determining the gas evolution parameters by thepresence of one or more minima in dI/dV in the increasing voltagedirection and the gas stopping points by the presence of one or moreminima in dI/dV in the decreasing direction of said voltage.
 12. Amethod as recited by claim 1, wherein said defect conditions determinedin step (j) comprise reverse connection of the battery leads to theapparatus, the measured open circuit voltage being less than apredetermined voltage, particularly -1 V.
 13. A method as recited inclaim 1, wherein said defect conditions, determined in step (j) compriseimproper connections of the battery to the apparatus, the measured opencircuit voltage being less than a predetermined voltage and higher thananother predetermined voltage.
 14. A method as recited by claim 1,wherein said defect conditions determined in step (j) comprise opencircuit voltage greater than a predetermined voltage, substantially zerocurrent flowing through the battery in response to an applied voltagesubstantially greater than the open circuit voltage.
 15. A method asrecited in claim 14, wherein said defect conditions determined in step(j) are at least one of the following defective conditions:(a) badconnections; (b) corroded terminals; (c) loss of electrolyte; and (d)bad relay type components in said apparatus.
 16. A method as recited byclaim 1, wherein said defect conditions determined in step (j) comprisesaid battery having cells mismatched in capacity said mismatch beingindicated by the presence of multiple gas evolution points in theincreasing current voltage direction and one or more gas stopping pointsin the decreasing current or voltage direction.
 17. A method as recitedby claim 1, wherein said defect conditions determined in step (j)comprise said battery having a defect caused by sulfated cell, saiddefect being indicated by the presence of one or more current-voltageinflections in the increasing current or voltage direction coupled withthe absence of corresponding gas stopping point in the decreasingcurrent or voltage direction.
 18. A method as recited in claim 1,wherein said defect conditions determined in step (j) comprise saidbattery having a defect caused by soft-shorted cell(s), said defectbeing indicated by a disproportionately low value for one of the Igas-upparameters or a jump or a minimum predetermined increased in opencircuit voltage due to step (b) in claim
 1. 19. A method as recited inclaim 1, wherein the internal resistance of the battery is determinedfrom two sets of Igas-up and Vgas-up parameters, each of said setscorresponding to different states of charge of said battery.
 20. Amethod as recited in claim 19 wherein said defect conditions areindicated by comparing the determined internal resistance with thecorresponding value characteristic of said battery size and capacitystored in memory.
 21. A method according to claim 1, wherein steps (b)and (e) comprise:(a) monotonously increasing the battery current over apredetermined period of time from zero or substantially zero current toa predetermined high limit current and holding said current at said highlimit value for a predetermined period of time and monotonouslydecreasing said current from said high limit value over a predeterminedperiod of time to zero or substantially zero current; (b) measuring theresponse battery voltage and/or impedance during said increasing anddecreasing current; (c) analyzing the data using the differential dV/dIvs. I or V, wherein I is the ucrrent and V is the voltage, or dZ/dI vs.I or V wherein I is the current, V is the voltage and Z is theimpedance; and (d) determining the gas evolution parameters by thepresence of one or more maxima in dV/dI or dZ/dI in the increasingcurrent direction and the gas stopping points by the presence of one ormore maxima in dV/dI or dZ/dI in the decreasing direction of saidvoltage.